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Re: mail relay was: Re: Intent to revive "expires" header from draft-ietf-mailext-new-fields-15

2008-07-25 11:08:47

Arnt Gulbrandsen wrote:

Jeff Macdonald writes:
Ok, you both lost me. Or Perhaps you are agreeing with me. How does a sending MTA know that final delivery has happened?

It only knows that final delivery will happen, not that it has happened.

It does not know that final delivery will happen.

An "MX" host is an incoming boundary MTA (see email-arch) for email. It typically does not know all of the valid addresses within the organization and it certainly cannot know about all of the possible errors that could prevent delivery, between the time the MX host takes receipt and final delivery finally happens (or doesn't.)

If you've delivered to the best MX, then that's as far as SMTP takes the message.

This, too, is often incorrect. Between the boundary MTA and final delivery can be multiple email hops, all using, or not using, SMTP.

The message may have a long way to go, but that's independent of SMTP and DELIVERBY. For example, the recipient may use something like Thunderbird offline, in which case the recipient's inbox at the server is just another spool, emptied when the recipient polls.

Folks seem to think that receipt by a host listed with an MX in the (presumably public) DNS is of fundamental import to for the end-to-end process of posting and delivering mail. It doesn't. It is merely one of a potentially large number of hops along the way. In fact, the email architecture has little or no technical details that pertain to the operation or semantics of a boundary MTA...



  Dave Crocker
  Brandenburg InternetWorking